Enabling High Activity Catalyst Co3O4@CeO2 for Propane Catalytic Oxidation via Inverse Loading

Propane catalytic oxidation is an important industrial chemical process. However, poor activity is frequently observed for stable C–H bonds, especially for non-noble catalysts in low temperature. Herein, we reported a controlled synthesis of catalyst Co3O4@CeO2–IE via inverse loading and proposed a strategy of oxygen vacancy for its high catalytic oxidation activity, achieving better performance than traditional supported catalyst Co3O4/CeO2–IM, i.e., the T50 (temperature at 50% propane conversion) of 217 °C vs. 235 °C and T90 (temperature at 90% propane conversion) of 268 °C vs. 348 °C at the propane space velocity of 60,000 mL g−1 h−1. Further investigations indicate that there are more enriched oxygen vacancies in Co3O4@CeO2–IE due to the unique preparation method. This work provides an element doping strategy to effectively boost the propane catalytic oxidation performance as well as a bright outlook for efficient environmental catalysts.


Introduction
Environmental pollution, especially atmospheric environment pollution, is becoming an increasingly serious problem. Volatile organic compounds (VOCs) emission gained widespread attention in very recent years [1][2][3]. The term VOCs means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which has become a main pollutant in the whole world. VOCs come from manifold sources, such as the petrochemical industry, construction materials, the printing industry and the electronics industry. In addition to the direct health effects, such as mutagenesis, teratogenesis and carcinogenesis, VOCs form 2.5 micrometer particulate matter (PM2.5) and damage the ozone layer through a series of photochemical reactions, thus seriously affecting the air quality around our living environment [4][5][6].
Propane, representing typical VOCs, is widely released from the energy activities involving liquefied natural gas (LNG), compressed natural gas (CNG) and liquefied petroleum gas (LPG), as well as other important industrial processes [7,8]. The effective treatment process of propane is catalytic oxidation for it provides high performance at a relatively low reaction temperature. However, there is still a problem in the catalytic oxidation of propane at low temperatures due to the stable C-H bonds in propane. Generally, precious metal (e.g., Au, Pd, Pt, Ru) catalysts show high activity in the propane catalytic oxidation [9][10][11][12]. Although noble metal catalysts, such as Pd and Pt, are supposed to be highly active and stable, their high expense, sintering rates, volatility, and the possibility of being poisoned by water or sulfur compounds limit their wide practical application. With

Structure and Morphology of the Obtained Samples
Two sets of composite samples with different loading directions, i.e., the Co3O4@CeO2-IE and Co3O4/CeO2-IM, and similar chemical compositions were synthesized (Table S1 in the Supporting Information (SI)). The morphology and hierarchical structure of the composites were characterized by a high-resolution transmission electron microscope (HRTEM) and energy dispersive spectrometer (EDS) elemental mapping analyses. The transmission electron microscope (TEM) images of Co(OH)2 show ultrathin nanosheets (about 10 nm) with clear lattice spacing of 0.26 nm ( Figure S2). And the typical HRTEM images (Figure 2a) of Co3O4@CeO2-IE shows the CeO2 and Co3O4 particles are uniformly distributed. As shown in Figure 2b, the typical HRTEM images clearly show the Co3O4 (311), (220) and CeO2 (111) with interlayer spacing of 0.243 nm, 0.288 nm and 0.310 nm, respectively. EDS elemental mapping further proves the homogeneous distribution, implying a sufficient contact between Ce and Co of the Co3O4@CeO2-IE catalyst. (Figure 2c). In comparison, Co3O4/CeO2-IM shows a heterogeneous distribution between Co and Ce, and the CeO2 is reunited, meaning that there is a weak interaction between Co3O4 and CeO2 (Figure 2d-f). These results suggest that Co3O4@CeO2-IE mainly exposes the CeO2 inserts into Co3O4 surface, while Co3O4/CeO2-IM mainly stands on the surface of Co3O4. And the particle size of Co3O4@CeO2-IE (10.8 nm) and Co3O4/CeO2-IM (41.8 nm) was calculated by Scherrer equation, indicating that the smaller particles could be fabricated by ion-exchange methods.

Structure and Morphology of the Obtained Samples
Two sets of composite samples with different loading directions, i.e., the Co 3 O 4 @CeO 2 -IE and Co 3 O 4 /CeO 2 -IM, and similar chemical compositions were synthesized (Table S1 in the Supporting Information (SI)). The morphology and hierarchical structure of the composites were characterized by a high-resolution transmission electron microscope (HRTEM) and energy dispersive spectrometer (EDS) elemental mapping analyses. The transmission electron microscope (TEM) images of Co(OH) 2 show ultrathin nanosheets (about 10 nm) with clear lattice spacing of 0.26 nm ( Figure S2). And the typical HRTEM images (Figure 2a) of Co 3 O 4 @CeO 2 -IE shows the CeO 2 and Co 3 O 4 particles are uniformly distributed. As shown in Figure 2b, the typical HRTEM images clearly show the Co 3 O 4 (311), (220) and CeO 2 (111) with interlayer spacing of 0.243 nm, 0.288 nm and 0.310 nm, respectively. EDS elemental mapping further proves the homogeneous distribution, implying a sufficient contact between Ce and Co of the Co 3 O 4 @CeO 2 -IE catalyst. (Figure 2c). In comparison, Co 3 O 4 /CeO 2 -IM shows a heterogeneous distribution between Co and Ce, and the CeO 2 is reunited, meaning that there is a weak interaction between Co 3 O 4 and CeO 2 (Figure 2d-   and CeO 2 (JCPDS:34-0394), further confirming the similarity in the effect of IE and IM methods on the preparation of the catalyst. However, there is a discrepancy in the intensity of the diffraction peaks. Co 3 O 4 /CeO 2 -IM has higher intensity of the diffraction peaks thanCo 3 O 4 @CeO 2 -IE, suggesting that the former has better crystallinity than the latter and the uniform distribution affects the crystallinity of Co 3 O 4 and CeO 2 . Notably, the characteristic peaks of Co 3 O 4 @CeO 2 -IE shift to lower 2θ values due to the diffusion of Ce 4+ and Co 2+ , Co 3+ into the opposite crystal lattice at the interface of Co 3 O 4 and CeO 2 while smaller Co 2+ (74.5 pm) and Co 3+ (61 pm) than Ce 4+ (97 pm) [33], thus forming a mixed phase. Co 3 O 4 /CeO 2 -IM has almost the same peak positions as Co 3 O 4 and CeO 2 , indicating that interactions between the Co 3 O 4 and CeO 2 seldom occur at the interface of Co 3 O 4 /CeO 2 -IM, consistent with the results of HRTEM analysis. The analysis results of nitrogen adsorption isotherms of the catalysts demonstrate that Co 3 O 4 @CeO 2 -IE (94.7 m 2 g −1 ) has a much higher BET surface area than Ni-CeO 2 -IM (24.3 m 2 g −1 ), meaning that the IE method can expose more active sites for the following catalytic applications (Figure 3b). The phase composition and crystallographic structure of Co3O4@CeO2-IE samples and Co3O4/CeO2-IM were examined by powder X-ray diffraction (PXRD). Figure 3a shows PXRD patterns Co3O4@CeO2-IE and Co3O4/CeO2-IM, which can be indexed to the composite phases of Co3O4 (JCPDS:42-1467) and CeO2 (JCPDS:34-0394), further confirming the similarity in the effect of IE and IM methods on the preparation of the catalyst. However, there is a discrepancy in the intensity of the diffraction peaks. Co3O4/CeO2-IM has higher intensity of the diffraction peaks thanCo3O4@CeO2-IE, suggesting that the former has better crystallinity than the latter and the uniform distribution affects the crystallinity of Co3O4 and CeO2. Notably, the characteristic peaks of Co3O4@CeO2-IE shift to lower 2θ values due to the diffusion of Ce 4+ and Co 2+ , Co 3+ into the opposite crystal lattice at the interface of Co3O4 and CeO2 while smaller Co 2+ (74.5 pm) and Co 3+ (61 pm) than Ce 4+ (97 pm) [33], thus forming a mixed phase. Co3O4/CeO2-IM has almost the same peak positions as Co3O4 and CeO2, indicating that interactions between the Co3O4 and CeO2 seldom occur at the interface of Co3O4/CeO2-IM, consistent with the results of HRTEM analysis. The analysis results of nitrogen adsorption isotherms of the catalysts demonstrate that Co3O4@CeO2-IE (94.7 m 2 g −1 ) has a much higher BET surface area than Ni-CeO2-IM (24.3 m 2 g −1 ), meaning that the IE method can expose more active sites for the following catalytic applications (Figure 3b).

Catalytic Oxidation Performance
The propane catalytic oxidation test was performed in the fixed bed reactor at WHSV of 60,000 mL⋅g −1 ⋅h −1 . The oxidative reactivity was evaluated by T50 and T90. Figure 4a shows that the Co3O4@CeO2−IE has much higher activity than Co3O4/CeO2-IM. Co3O4@CeO2−IE as it participates in the propane catalytic oxidation reaction, with T50 of 217 °C and T90 of 268 °C, significantly superior to T50 of 235 °C and T90 of 348 °C for Co3O4/CeO2-IM catalyst. The activation energies (Ea) were evaluated according to the Arrhenius plots in Figure 4b, demonstrating a much lower Ea value of 63.7 kJ⋅mol −1 for Co3O4@CeO2−IE than Co3O4/CeO2−IM (89.5 kJ⋅mol −1 ). And Co3O4@CeO2-IE shows a superior activity toward the total oxidation of propane to Co3O4/CeO2-IM, and its reaction rate at 235 °C is 9.80 × 10 -7 mol g -1 s -1 , is almost 1.5 times that of Co3O4/CeO2-IM (6.91 × 10 -7 mol g -1 s -1 ). These findings confirm that Co3O4@CeO2−IE exhibits higher catalytic activity (Table S2).

Catalytic Oxidation Performance
The propane catalytic oxidation test was performed in the fixed bed reactor at WHSV of 60,000 mL·g −1 ·h −1 . The oxidative reactivity was evaluated by T 50 and T 90 . Figure 4a shows that the Co 3 O 4 @CeO 2 −IE has much higher activity than Co 3 O 4 /CeO 2 -IM. Co 3 O 4 @CeO 2 −IE as it participates in the propane catalytic oxidation reaction, with T 50 of 217 • C and T 90 of 268 • C, significantly superior to T 50 of 235 • C and T 90 of 348 • C for Co 3 O 4 /CeO 2 -IM catalyst. The activation energies (E a ) were evaluated according to the Arrhenius plots in Figure 4b, demonstrating a much lower Ea value of 63.7 kJ·mol −1 for Co 3 O 4 @CeO 2 −IE than Co 3 O 4 /CeO 2 −IM (89.5 kJ·mol −1 ). And Co 3 O 4 @CeO 2 -IE shows a superior activity toward the total oxidation of propane to Co 3 O 4 /CeO 2 -IM, and its reaction rate at 235 • C is 9.80 × 10 −7 mol g −1 s −1 , is almost 1.5 times that of Co 3 O 4 /CeO 2 -IM (6.91 × 10 −7 mol g −1 s −1 ). These findings confirm that Co 3 O 4 @CeO 2 −IE exhibits higher catalytic activity (Table S2).

Surface Chemistry Analysis of the Catalysts
To gain deep insight into the origin of differences in catalytic behaviors between Co 3 O 4 @CeO 2 -IE and Co 3 O 4 /CeO 2 -IM, X-ray photoelectron spectroscopy (XPS) analysis was used to access the electronic state of the catalyst. The Ce 3d XPS spectra for the two catalysts were performed. And the spectra were deconvoluted into ten sub−peaks. The peaks denoted as U and V are individually attributed to Ce 3d 5/2 and Ce3d 3/2 , respectively. Among those ten peaks, U 0 , U I , V 0 and V I are assigned to Ce 3+ (red line) and the rest of them are related to Ce 4+ (blue line) [34]. As depicted in Figure 5a, the XPS results reveal that the Ce 3+ fraction (49.6%) on the surface of Co 3 O 4 @CeO 2 -IE is nearly twice more than that of Co 3 O 4 /CeO 2 -IM (28.6%). It is proposed that there are more oxygen defects on the surface of Co 3 [35,36]. The relative oxygen vacancy (O v ) proportion on the surface of Co 3 O 4 @CeO 2 -IE (40.7%) is much higher than that of Co 3 O 4 /CeO 2 -IM (25.0%). The peaks at 780.5 eV and 795.8 eV are attributed to the Co 2p 1/2 and Co 2p 3/2 core line of Co 2+ , respectively, whereas those at 779.2 eV and 794.2 eV are related to Co 3+ [37]. The ratio of Co 2+ /Co 3+ (2.64) in Co 3 O 4 @CeO 2 -IE is much higher than that (1.96) in Co 3 O 4 /CeO 2 −IM due to the fact that the oxygen defects of the catalyst can promote the reduction in metal ions (Figure 5c and Table S3).

Catalytic Oxidation Performance
The propane catalytic oxidation test was performed in the fixed bed reactor at WHSV of 60,000 mL⋅g −1 ⋅h −1 . The oxidative reactivity was evaluated by T50 and T90. Figure 4a shows that the Co3O4@CeO2−IE has much higher activity than Co3O4/CeO2-IM. Co3O4@CeO2−IE as it participates in the propane catalytic oxidation reaction, with T50 of 217 °C and T90 of 268 °C, significantly superior to T50 of 235 °C and T90 of 348 °C for Co3O4/CeO2-IM catalyst. The activation energies (Ea) were evaluated according to the Arrhenius plots in Figure 4b, demonstrating a much lower Ea value of 63.7 kJ⋅mol −1 for Co3O4@CeO2−IE than Co3O4/CeO2−IM (89.5 kJ⋅mol −1 ). And Co3O4@CeO2-IE shows a superior activity toward the total oxidation of propane to Co3O4/CeO2-IM, and its reaction rate at 235 °C is 9.80 × 10 -7 mol g -1 s -1 , is almost 1.5 times that of Co3O4/CeO2-IM (6.91 × 10 -7 mol g -1 s -1 ). These findings confirm that Co3O4@CeO2−IE exhibits higher catalytic activity (Table S2).

Surface Chemistry Analysis of the Catalysts
To gain deep insight into the origin of differences in catalytic behaviors between Co3O4@CeO2-IE and Co3O4/CeO2-IM, X-ray photoelectron spectroscopy (XPS) analysis was used to access the electronic state of the catalyst. The Ce 3d XPS spectra for the two catalysts were performed. And the spectra were deconvoluted into ten sub−peaks. The peaks denoted as U and V are individually attributed to Ce 3d5/2 and Ce3d3/2, respectively. Among those ten peaks, U0, U I , V0 and V I are assigned to Ce 3+ (red line) and the rest of them are related to Ce 4+ (blue line) [34]. As depicted in Figure 5a, the XPS results reveal that the Ce 3+ fraction (49.6%) on the surface of Co3O4@CeO2-IE is nearly twice more than that of Co3O4/CeO2-IM (28.6%). It is proposed that there are more oxygen defects on the surface of Co3O4@CeO2−IE (Ce 4+ + OL → Ce 3+ + OV) [35,36]. The relative oxygen vacancy (Ov) proportion on the surface of Co3O4@CeO2-IE (40.7%) is much higher than that of Co3O4/CeO2-IM (25.0%). The peaks at 780.5 eV and 795.8 eV are attributed to the Co 2p1/2 and Co 2p3/2 core line of Co 2+ , respectively, whereas those at 779.2 eV and 794.2 eV are related to Co 3+ [37]. The ratio of Co 2+ /Co 3+ (2.64) in Co3O4@CeO2-IE is much higher than that (1.96) in Co3O4/CeO2−IM due to the fact that the oxygen defects of the catalyst can promote the reduction in metal ions (Figure 5c and Table S3). The structures of oxygen vacancies were further investigated by electron paramagnetic resonance (EPR). Figure 6a,b show that the two catalysts have the g value of 2.004, which can be attributed to the unpaired electrons trapped in the OV in the Co3O4/CeO2 materials. Co3O4@CeO2−IE has higher intensity of the peaks than Co3O4/CeO2-IM. The intensity of the peaks, which is associated with the number/density of the Ov, implies that Co3O4@CeO2−IE have more Ov.
The surface defects were further examined by the Raman spectroscopy (Figure 6c). Co3O4/CeO2−IM exhibits the signals of 191 (F2g 1 ), 474 (Eg), 516 (F2g 2 ), 615 (F2g 3 ) and 680 cm −1 (A1g), which correspond to pure Co3O4 [38], indicating that Ce seldom interacts with Co3O4 in Co3O4@CeO2−IE (in accordance with the results of HRTEM and XRD analysis). All of the bands should be mainly assigned to the vibration mode of Co3O4. The sharp band at 461 cm −1 (F2g band) of CeO2 can be assigned to the vibration mode of CeO2 fluorspar structure ( Figure S3) [39,40]. The F2g band of Co3O4@CeO2−IE shows a red−shift of 16 cm −1 to 445 cm −1 with a sharp peak, which can be attributed to the Co-O-Ce bond induced by residual stress or lattice distortion in CeO2 structure, further suggesting a strong interaction between Co3O4 and CeO2 in Co3O4@CeO2−IE [41].
O2−TPD profiles were determined to recognize the oxygen species desorbed from the surface as a function of temperature. Figure 6d  The structures of oxygen vacancies were further investigated by electron paramagnetic resonance (EPR). Figure 6a, Figure S3) [39,40]. The F 2g band of Co 3 O 4 @CeO 2 −IE shows a red−shift of 16 cm −1 to 445 cm −1 with a sharp peak, which can be attributed to the Co-O-Ce bond induced by residual stress or lattice distortion in CeO 2 structure, further suggesting a strong interaction between Co 3 O 4 and CeO 2 in Co 3 O 4 @CeO 2 −IE [41].
O 2 −TPD profiles were determined to recognize the oxygen species desorbed from the surface as a function of temperature. Figure 6d shows more obvious adsorption (225 • C-600 • C) and (725 • C-950 • C) of oxygen species in Co 3 O 4 @CeO 2 −IE than in Co 3 O 4 @CeO 2 −IM, indicating more O V . The quantitative analysis shows that Co 3 O 4 @CeO 2 −IE has nearly three times higher desorptive capacity of O (0.0838 mmol g −1 ) than Co 3 O 4 /CeO 2 −IM (0.0284 mmol g −1 ). The defects O V caused by Ce doping can strengthen the adsorption, activation ability of oxygen molecules and surface oxygen species migration ability, resulting in the generation of abundant active oxygen species at low temperatures.
The above analysis results indicate that the Ce 3+ ions are partly exchanged with the Co 2+ ions in Co(OH) 2 , leading to a special interface between Co(OH) 2 and Ce(OH) 4 with the diffusion of Ce 4+ and Co 2+ into opposite crystal phases and thus more O v of the interface in Co 3 O 4 @CeO 2 −IE via inverse loading. There existed the lattice distortion due to the radius of Co 2+ or Co 3+ being smaller than Ce 4+ and the charge imbalance for Co 2+ or Co 3+ and Ce 4+ in

Catalytic Mechanism and Density−Functional Theory (DFT) Calculations
Marse−van Krevelen (MvK) mechanism is suitable for most non−metal catalysts to oxidize VOCs. MvK mechanism is based on redox reaction. Its essence is that the lattice oxygen in the catalyst oxidizes VOCs. In this reaction process, firstly, when VOCs react with lattice oxygen and generate oxygen vacancies, the metal oxides are reduced consequently. Secondly, oxygen vacancies are filled by oxygen in the air. The adsorption capacity and oxygen transfer capacity are the key contributing factors of the MvK mechanism. It is widely accepted that the dissociative adsorption of C3H8 on the catalyst surfaces triggers the catalytic oxidation process [42,43]. We calculated DFT in order to further elucidate the adsorption mechanism of Co3O4@CeO2−IE. To simulate the sample, a Co3O4−(311) surface model was created to expose the most crystal faces in the experiments. The Co3O4−(311) surface, which contains OV (Co-OV) adjacent to Co atoms, was simulated first, and then Co3O4@CeO2−IE was simulated by replacing Co atoms with Ce atoms in Co3O4. In the simulations, VO is adjacent to both Co and Ce atoms (Co-OV-Ce). The reaction is initiated by the adsorption of propane to the surface, followed by the dehydrogenation of propane to produce free radicals, which are finally oxidized to CO2 and H2O, where the desorption of generated propane radicals from the surface of the catalyst is the rate−limiting step for the entire reaction. Hence, the adsorption energies of propane adsorption are studied. Two substrate structures for adsorption are shown in Figure 7a,b. The calculated adsorption free energies (ΔEads) of propane on the two substrates are 0.17 eV and −0.85 eV, respectively. That indicates that propane is not favorable for adsorption on the Co-OV substrate, but is favorable for adsorption on the Co-OV-Ce surface. This is mainly due to the fact that Ce atoms have a greater number of outer electrons than Co atoms, and Ce doping in the system not only induces a large number of O vacancy defects, but also increases the free electrons of the system. The

Catalytic Mechanism and Density−Functional Theory (DFT) Calculations
Marse−van Krevelen (MvK) mechanism is suitable for most non−metal catalysts to oxidize VOCs. MvK mechanism is based on redox reaction. Its essence is that the lattice oxygen in the catalyst oxidizes VOCs. In this reaction process, firstly, when VOCs react with lattice oxygen and generate oxygen vacancies, the metal oxides are reduced consequently. Secondly, oxygen vacancies are filled by oxygen in the air. The adsorption capacity and oxygen transfer capacity are the key contributing factors of the MvK mechanism. It is widely accepted that the dissociative adsorption of C 3 H 8 on the catalyst surfaces triggers the catalytic oxidation process [42,43]. We calculated DFT in order to further elucidate the adsorption mechanism of Co 3 O 4 @CeO 2 −IE. To simulate the sample, a Co 3 O 4 −(311) surface model was created to expose the most crystal faces in the experiments. The Co 3 O 4 −(311) surface, which contains O V (Co-O V ) adjacent to Co atoms, was simulated first, and then Co 3 O 4 @CeO 2 −IE was simulated by replacing Co atoms with Ce atoms in Co 3 O 4 . In the simulations, VO is adjacent to both Co and Ce atoms (Co-O V -Ce). The reaction is initiated by the adsorption of propane to the surface, followed by the dehydrogenation of propane to produce free radicals, which are finally oxidized to CO 2 and H 2 O, where the desorption of generated propane radicals from the surface of the catalyst is the rate−limiting step for the entire reaction. Hence, the adsorption energies of propane adsorption are studied. Two substrate structures for adsorption are shown in Figure 7a,b. The calculated adsorption free energies (∆E ads ) of propane on the two substrates are 0.17 eV and −0.85 eV, respectively. That indicates that propane is not favorable for adsorption on the Co-O V substrate, but is favorable for adsorption on the Co-O V -Ce surface. This is mainly due to the fact that Ce atoms have a greater number of outer electrons than Co atoms, and Ce doping in the system not only induces a large number of O vacancy defects, but also increases the free electrons of the system. The electronic density of Ce in the Co 3 O 4 @CeO 2 −IE system near the Fermi level can be analyzed according to the density of states in Figure 7c,d, conductive to electron exchange with propane. The density of states of the Co-O V -Ce structure suggests that the additional electrons provided by the Ce atoms are mainly populated above the Fermi energy level 0.3-0.7 eV above the Fermi energy level. In order to better investigate the adsorption property of propane by the extra electrons from Ce atoms, we calculated the crystal orbital Hamilton population (COHP) between Ce and H after adsorption of propane ( Figure S4). We found that the extra electrons provided in the Ce atoms coupled with the H atoms favor the adsorption and dehydrogenation of propane [44]. As shown in the charge density difference in Figure 7e,f, the presence of Ce atoms leads to a greater exchange of electron density between the substrate and propane. When Ce fully invades the surface, Co 3 O 4 @CeO 2 −IE has more Co-O V -Ce structures than Co 3 O 4 /CeO 2 −IM [45], and is more conducive to the decomposition of propane.
Molecules 2023, 28, x FOR PEER REVIEW 8 of 12 propane. The density of states of the Co-OV-Ce structure suggests that the additional electrons provided by the Ce atoms are mainly populated above the Fermi energy level 0.3-0.7 eV above the Fermi energy level. In order to better investigate the adsorption property of propane by the extra electrons from Ce atoms, we calculated the crystal orbital Hamilton population (COHP) between Ce and H after adsorption of propane ( Figure S4). We found that the extra electrons provided in the Ce atoms coupled with the H atoms favor the adsorption and dehydrogenation of propane [44]. As shown in the charge density difference in Figure 7e,f, the presence of Ce atoms leads to a greater exchange of electron density between the substrate and propane. When Ce fully invades the surface, Co3O4@CeO2−IE has more Co-OV-Ce structures than Co3O4/CeO2−IM [45], and is more conducive to the decomposition of propane.

Discussion
In summary, we successfully fabricated a new kind of Co3O4@CeO2-IE catalyst by ion−exchange method. The obtained Co3O4@CeO2-IE presents superior catalytic activity in the propane catalytic oxidation. The results of XPS, EPR, Raman spectra and DFT calculations etc., indicate that Co3O4@CeO2-IE catalyst possesses abundant oxygen vacancies on the surface of Co3O4-CeO2, which adsorb the propane. This study not only presents a new kind of non-noble metal catalyst for efficient catalytic oxidation of propane, but also highlights a strategy for the design of the oxygen vacancy for an advanced catalyst.

Discussion
In summary, we successfully fabricated a new kind of Co 3  on the surface of Co 3 O 4 -CeO 2 , which adsorb the propane. This study not only presents a new kind of non-noble metal catalyst for efficient catalytic oxidation of propane, but also highlights a strategy for the design of the oxygen vacancy for an advanced catalyst. All chemicals were purchased from Aladdin, China, and used as received. Synthesis of Co(OH) 2 nanosheets. According to our previously reported hybridization route [46], the synthetic process for the Co(OH) 2 was as follows: Commercial MgCO 3 was calcined at 750 • C for two hours to obtain MgO. Then, the resultant MgO was put into distilled water with a MgO−to−H 2 O mass ratio of 1:10 under stirring for 24 h. Finally, the white Mg(OH) 2 product was separated by filtration, washed with deionized water and absolute ethanol three times and dried at room temperature. The resultant Mg(OH) 2 (0.73 g equivalent to 0.0125 mol) was added to 25 mL of an aqueous solution containing 0.0125 mol of Co(NO 3 ) 2 ·6H 2 O. After stirring vigorously for two hours at room temperature, the green Co(OH) 2 product was separated by filtration, washed with deionized water and ethyl alcohol three times and dried at room temperature overnight.

Preparation of Co 3 O 4 @CeO 2 -IE Catalyst
The Co 3 O 4 @CeO 2 -IE surrounded catalyst was synthesized by ion−change method. The fresh 1.86 g Co(OH) 2 was added into the 40 mL solution including of 0.22 g Ce(NO 3 ) 3 ·6H 2 O and then stirred under room temperature for 30 min. And thus put the mixture into Teflon−lined stainless steel autoclave, sealed, and maintained at 120 • C for 12 h. When cooled to the room temperature, the yellow-greenish products of Co(OH) 2 /Ce(OH) 4 were separated by filtration, washed with deionized water and ethanol six times, and dried at 80 • C overnight. The hydroxide products were calcinated at 300 • C for 2 h in muffle furnace denoted as Co 3 O 4 @CeO 2 -IE. The Co loading is 45.3 wt % and the Ce loading is 8.36 wt % which was determined by ICP.

Preparation of Co 3 O 4 /CeO 2 -IM Catalyst
The Co 3 O 4 /CeO 2 −IM catalyst was synthesized by wet impregnation method. CeO 2 support was prepared by reference method. 5.82 g Co(NO 3 ) 2 ·6H 2 O was added into deionized 5 mL water solution and then added 0.09 g CeO 2 into the mixture. The slurry was evaporated under stirring at 110 • C until dried thoroughly. And then followed the calcination denoted as Co 3 O 4 /CeO 2 −IM. The Co loading was 45.4 wt % and the Ce loading was 8.38 wt % which was determined by ICP and the similar content as Co 3 O 4 @CeO 2 -IE.

Characterizations
The morphology was characterized using a Hitachi S−4800 scanning electron microscope (SEM). The chemical composition of the solids was determined by an inductively coupled plasma−atomic core line spectroscopy (ICP-AES) (Thermo Fisher iCAP PRO (OES)). XRD patterns were performed on a Rigaku Miniflex 600 using Ni−filtered Cu Ka radiation (k = 0.15408 nm) at 40 Kv and 40 mA, and the scope of data collection was 2θ = 10-80 • . The HRTEM images were obtained using a FEI Talos F200X G2 electron microscope operated at 200 kV. Results of element mapping were obtained on a super-x equipped with an Energy Dispersive Spectrometer (EDS). Brunauer-Emmett-Teller (BET) surface area measurement was conducted on a ASAP 2460 instrument. Before the BET surface area measurement, the samples were dried at 300 • C for 4 h under vacuum. Temperature−programmed desorption of O 2 (O 2 −TPD) was performed on a Tianjin XQ TP−5080B chemisorption instrument with a thermal conductivity detector (TCD). The sample (100 mg) was first at 300 • C for 1 h to remove moisture under the steam (30 mL/min). After cooling to room temperature, 5 vol% O 2 /N 2 mixture was switched on with a flow rate of 25 mL·min −1 at 50 • C for 1 h, and then cooled down to room temperature in the oxidizing atmosphere. Then continuously purging in a He flow for 30 min, the measurement started from room temperature to 950 • C at a heating rate 10 • C·min −1 after the baseline of single was stable. Electron paramagnetic resonance (EPR, Bruker A300, Bremen, Germany) were tested by a FA−200 (JES) electron paramagnetic resonance spectrometer. Raman spectra were carried out on Renishaw inVia Qontor with 532 nm of incident light. X−ray photoelectron spectra (XPS) were recorded on Escalab 250Xi using Al Kα radiation (1486.6 eV, 150 W) with binding energies (BEs) calibrated against the C1s peak of adventitious carbon at 284.8 eV.

Catalyst Evaluation
Propane oxidation test was carried out in a fixed-bed reactor using reactant of 0.5 vol% C 3 H 8 and 21 vol% O 2 , balanced with N 2 . The Weight-Hourly-Space-Velocity (WHSV) was set at 60,000 mL·g −1 ·h −1 with reaction temperature increased from room temperature to 500 • C (5 • C/min). The products were in situ tested by a gas chromatograph (Agilent, GC-7890B, Santa Clara, CA, USA). C 3 H 8 conversion was acquired from the following formulas: To obtain the information of the apparent activation energy (E a ), the reaction was controlled at the kinetic regime (C 3 H 8 conversion is under 10%). The equation below was used for the calculation of E a by acquiring the slope: where k represents the reaction rate constant, T is the absolute temperature and R stands for the gas constant.

Computational Details
First−principles calculations were carried out using the projector−augmented wave method implemented in the Quantum ESPRESSO based on density functional theory (DFT) [47,48]. The Perdew-Burke-Ernzerhof (PBE) functional of the generalized gradient approximation (GGA) was adopted for electron exchange and correlation interaction [49]. The van der Waals interactions between layers were corrected using the DFT−D3 functional [50]. The ions relaxation was achieved until the force for per atom was less than 0.02 eV/Å and the total energy converged to 10 −5 eV. A vacuum spacing of 15 Å was used to prevent interaction between adjacent slabs. The change in free energy for the adsorption (∆E ads ) of the target by the catalyst substrate is defined by E q : ∆E ads = Etotal -Esubstrate -Etarget, where Etotal is the total energy of the catalyst substrate and propane, Esubstrate and Etarget are the energies of the substrate and free target molecule, respectively.